The use of cyclopentadienyl metal complexes, in particular the use of metallocene complexes in a mixture with activating co-catalysts, preferably alumoxanes (MAO), for the polymerization of olefins and diolefins has been known for a long time (e.g. EP-A 129 368, 347 128, 347 129, 69 951, 351 392, 485 821, 485 823).
The metallocenes have proven to be highly effective, specific catalysts in the polymerization of in particular olefins. In order, therefore, to increase the activity, selectivity, control of the micro-structure, the molecular weights and the molecular weight distribution, a large number of novel metallocene catalysts or metallocene catalyst systems have been developed in recent years for the polymerization of olefinic compounds.
The MAO-based catalyst systems described above have serious drawbacks, however, as will be explained in detail below. Firstly, aluminoxanes, in particular MAO, can be manufactured with high reproducibility neither in situ nor during preforming. MAO is a mixture of various aluminum alkyl-containing species, which are present in equilibrium with each other. The number and the structure of the aluminum compounds occurring in MAO is not defined precisely. The polymerization of olefins with catalyst systems that contain MAO is therefore not always reproducible. Moreover, MAO is not storable over long periods and its composition changes under thermal stress. A major disadvantage is the high surplus of MAO which is required for the activation of metallocenes. The large MAO/metallocene ratio is an essential pre-requisite for obtaining high catalyst activities. This results in a critical process drawback, as the aluminum compound has to be separated from the polymer during the working up. MAO is furthermore a cost-determining factor. High MAO surpluses are uneconomic for industrial application.
In order to circumvent these drawbacks, alumoxane-free polymerization catalysts have been developed in recent years. For example, Jordan et al report in J. Am. Chem. Soc., Vol. 108 (1986), 7410 on a cationic zirconocene-methyl complex which possesses tetraphenyl borate as counter-ion and polymerizes ethylene into methylene chloride. In EP-A 277 003 and EP-A 277 004 ionic metallocenes are described which are produced by the reaction of metallocenes with ionizing reagents. In EP-A 468 537 catalysts of ionic structure are described which are obtained by reacting metallocene-dialkyl compounds with tetrakis(pentafluorophenyl)boron compounds. The ionic metallocenes are suitable as catalysts for the polymerization of olefins. A disadvantage is however the great sensitivity of the catalysts to impurities such as e.g. humidity and oxygen. Measures therefore have to be taken during the performance of polymerizations to guarantee as great a purity of the monomers and solvents used as possible. This is very complicated technically and expensive.
In order to overcome these drawbacks, there are described in EP-A 427 697 and in WO 92/01723 processes for the polymerization of olefins in which a combination of metallocene dichlorides with aluminum alkyls and tetrakis(pentafluorophenyl)boron compounds is used as a catalyst system. The aluminum alkyl compounds on the one hand serve as alkylation agents of the metallocene component and on the other function as scavengers in order to protect the active catalyst species against impurities.
The methods corresponding to the prior art for preparing the cationic metallocenes have the drawback, however, that the cationizing reagents, e.g. tetrakis(pentafluorophenyl)boron compounds, are difficult to synthesize in some cases and their use is cost-intensive.
According to Bercaw et al., JACS (1972), 94, 1219, there is obtained by thermolysis of bis(.eta..sup.5 -pentamethylcyclopentadienyl)titanium dimethyl the fulvene complex (.eta..sup.6 -2,3,4,5-tetramethylcyclopentadienyl-1-methylene)(.eta..sup.5 -pentamethylcyclopenta-dienyl)-titanium methyl. Nothing is known about the polymerization activity of this complex. In T. J. Marks et al., JACS (1988), 110, 7701 the thermolysis of pentamethyl-cyclopentadienyl complexes of zirconium and hafnium is described. By thermolysis of bis(.eta..sup.5 -pentamethyl-cyclopentadienyl)zirconium diphenyl the fulvene complex (.eta..sup.6 -2,3,4,5-tetra-methylcyclopentadienyl-1-methylene)(.eta..sup.5 -pentamethylcyclopentadienyl)-zirconium phenyl is obtained. This compound is not polymerization-active on its own.